A common problem associated with liquid sample analytical measurement instruments is the carryover of an injected sample from run-to-run. Whenever two samples are injected sequentially, there is always some trace amount of the first injected sample left in the cell when a second injected sample is measured. Any number of analytical instruments may suffer from this sample carryover, among these are instruments to measure light scattering, refractive index, ultraviolet absorption, viscosity, and electrophoretic mobility. Of particular interest to the present invention is the field of electrophoretic mobility measurements, such as those discussed by Hsieh and Trainoff in U.S. Pat. No. 8,441,638 issued May 14, 2013 and incorporated herein by reference. Various techniques have been used to mitigate the problem of sample carryover. The most simple and commonly used methods to overcome this contamination problem include flushing a large volume of cleaning fluid between sample injections and injecting large amounts of the samples to insure that the cell is overfilled many times such that the majority of the previously injected sample is flushed from the measurement chamber prior to the measurement of the incoming sample. However this is often only marginally effective as measurement cells often include complicated internal geometries with regions that are poorly connected to the main volume of the measurement cell such that it is easy to push sample into these regions but difficult to flush the sample out again. These processes can also waste time and valuable sample and frequently produce excess waste.
One of the most problematic geometries contained within analytical instruments that give rise to sample carryover issues are o-ring grooves. O-rings are commonly used in measurement cells because they are effective, inexpensive, durable and reliable. Moreover a cell built with o-ring seals can generally be disassembled for cleaning and to replace worn or damaged components. O-rings, however, are notorious for enabling the trapping of sample. Consider the uncompressed standard face seal o-ring groove design as shown in FIG. 1. In a standard face seal design, the top plate 101, in this case a disc shaped window, is pressed directly against the manifold surface 102 which contains the groove 103 with no gap between the manifold surface 102 and top plate 101 after compression. The vertical extent of the groove is smaller, typically by 15%, than the diameter of the o-ring 104. As the o-ring 104 is compressed, it deforms and spreads laterally within the groove 103, which is purposefully oversized. The horizontal extent of the groove 103 is typically 1.5× the o-ring diameter, although the width and compression can be optimized for particular applications. The extra volume within the groove 103 is present to compensate for worst-case tolerances of the o-ring, groove machining, chemical swelling of the o-ring material, and differential thermal expansion of the o-ring material and the groove over its operational temperature range. An o-ring designer is forced to leave a substantial extra gap in the groove 103 so that the designed measurement cell does not leak given worst-case tolerances. This gap acts as a carryover reservoir, or dead volume, for any fluid entering the cell, and since the window 101 is pressed directly against the manifold top surface 102, once sample is pushed into this space, it is very difficult to remove. When the seal is pressurized, the o-ring stretches and is forced to the outside edge of the groove, causing the interior gap to fill with fluid. When the seal is depressurized, friction can hold the o-ring in its stretched configuration and the fluid trapped in the seal remains. Given a static load, the only means by which the trapped sample may exit the groove is by diffusion, which is a slow process. An objective of the present invention is to enable a more rapid method than simple diffusion to drastically reduce or completely eliminate carryover in a measurement cell from one sample injection to the next.
Since the interior of the measurement cell is pressurized relative to the environment, the design rules teach that the most reliable seal is formed when the o-ring is pressed against the outside wall 105 of the groove 103, to minimize stretching, and the corresponding reduction in seal compression that results. Nevertheless, one might be tempted to address the carryover problem by designing the o-ring groove 103 so that the ring hugs the inner wall 106, making sure that the dead volume is outside the sample space. This works so long as the pressurization of the cell interior that invariably accompanies filling the cell is small enough that the o-ring's tensile strength, coupled with friction between the o-ring and the sealing surfaces, is sufficient to keep the o-ring in place. However, since o-rings are usually made of compliant rubber, this is rarely sufficient. Moreover as the o-ring ages, repeated pressurizations will cause it to creep to the outside of the groove and friction will hold it in place. At this point the advantages of an inner wall hugging o-ring design are overcome because we once again have the dead volume in the interior of the cell, and in addition, now we have a stretched seal with reduced compression.
As discussed above, some analytical instruments may be engineered to incorporate optimized groove designs which minimize the groove width, using as input parameters: the o-ring material, the expected temperature range of operation, material tolerances, and chemical swell. At extreme tolerances, these grooves may have zero dead volume. However, while these “optimized” systems may enable a decrease of groove volume, and thus less dead volume, if any of the tolerances is exceeded, the system risks a catastrophic failure such as leakage or window breakage. It is an objective of the present invention to enable a means by which carryover may be minimized or eliminated in systems containing o-ring grooves so as to allow for significant non-idealities, including systems wherein the o-ring groove comprises a significant dead volume.